Fifteen states have submitted fewer than 100 total mental health records to the federal government’s instant check system, marking what gun control advocates said is a major flaw in the system.

Since the National Instant Criminal Background Check System (NICS) became operational in 1998, the 15 lowest-performing states have reported fewer than 100 records between them, according to a new report from advocacy group Mayors Against Illegal Guns.

The spotlight has returned to mental health and gun crimes in the wake of a review this week that found the Sandy Hook shooter had mental problems, and after a man with mental health issues assaulted his politician father in a high-profile case in Virginia earlier this month.

“Everyone understands that mental health plays a role in some shootings,” said David Chipman, a former agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives and former adviser to the mayors’ group. “It seems like there has been some progress made, but there’s still a lot of work that needs to be done.”

The report found that of those 15 bottom-performing states, only Nebraska and North Dakota had laws that require or permit the records to be turned over.

Amid the disputes on guns, advocates on both sides of the issue generally agree that better medical documentation could flag people at risk of doing harm to themselves or others.

Those efforts got a boost this week after authorities in Connecticut released a report saying the shooter in Newtown last year, Adam Lanza, was obsessed with violence and mass shootings, and had easy access to guns that were legally purchased by his mother.

But the system can only take you so far, said John Hudak, a fellow in governance studies at the Brookings Institution.

“If Connecticut’s reporting of their mental health records was perfect, it wouldn’t have stopped what happened in Newtown,” Mr. Hudak said.

However, Mr. Hudak cautioned that gun violence remains a social problem that cannot be solved only in people’s homes or by better parenting.

Federally licensed gun dealers are required to perform NICS checks when they sell a gun. A bill to expand the required checks to private sales online and at gun shows failed earlier this year in the U.S. Senate, though a number of states have passed new gun control and background check laws in the wake of last year’s Connecticut school shootings.

Under the current NICS system, states voluntarily provide records to the federal government. The report, however, found that most of the 15 top-performing states received federal grant money from 2009 to 2012 to improve collection and submission of the relevant records.

Pennsylvania, New Jersey, Utah, Florida and Maine had the largest increases in reported records from October 2012 to May 2013. Florida also had the largest percent increase in mental health submissions over their previous total, jumping 82 percent from 49,903 to 90,824.

Virginia has been one of the most diligent in turning over records to the federal government, especially in the wake of reforms instituted after the Virginia Tech mass shooting in 2007.

Nevertheless, issues inside the state persist, and the specific issue of mental health treatment has gotten particularly close attention in recent weeks after Austin “Gus” Deeds, 24, stabbed state Sen. R. Creigh Deeds before taking his own life.

Mr. Deeds, a former Democratic gubernatorial nominee, sharply criticized the state’s mental health system this week; Gus Deeds was given a psychiatric evaluation the day before the tragedy occurred, but was turned away after the Rockbridge Community Services Board could not find him a bed.

Several nearby facilities reported later that they had available space but were not contacted.